1. Field of the Invention
The present invention is directed generally to a thermal ink jet printhead. More specifically, the present invention is directed to a passivation layer for a thermal ink jet printhead.
2. Background of the Invention
In a thermal ink jet (TIJ) printhead, ink is projected through an orifice by the repetitive high speed collapse of a vapor bubble created by the resistive heating of a resistor. The implosion of the bubbles can erode the surfaces of the TIJ printhead. This erosion, alternatively called cavitation, can cause failure of jet producing elements, such as a resistor in a thermal ink jet printhead, a protective overcoat, or an underlying substrate. This deleterious effect can be mitigated by a passivation layer covering the area susceptible to cavitation.
An ideal passivation layer for TIJ resistors is resistant to the mechanical stresses during bubble collapse, has a smooth surface topography for a consistent bubble nucleation, and is chemically inert to withstand various operating environments including high and low pH levels from various kinds of inks. Prior improvements in the life expectancy of TIJ printheads have been achieved by the choice of geometry, the materials, and the fluid over the resistor. For example, co-assigned U.S. Pat. No. 4,528,574, the disclosure of which is incorporated herein by reference, uses an acoustic absorber in a TIJ printhead to reduce damage from cavitation.
Traditionally Ta has been used as the top passivation layer material to protect the TaAl resistors from the cavitation damage. However, Ta and Ta alloys suffer from several characteristics that deleteriously impact performance in a thermal ink jet printhead environment.
It is known that to be effective, boiling must occur very reproducibly when heat flux or temperature reach a certain level. A surface that is changing due to, for example, cavitation or corrosion, suffers from a deficiency in stable nucleation sites on the surface. However, known passivation layer materials have not sufficiently resisted cavitation and corrosion over extended use, resulting in a dynamically changing surface topography and reduced performance.
Cavitation remains an industry problem and negatively impacts the life of TIJ printheads. The problems from cavitation are especially acute for large arrays of jets which are more expensive to manufacture and are statistically more prone to failure.
In addition improvements in TIJ technology, such as a semi-permanent TIJ printhead, require improved resistor reliability. The adoption of a high resistivity resistor, with its accompanying higher voltage, also demands stronger passivating materials to prevent arcing that could arise if a crack exists in the dielectric between the resistor and a metallic overcoat.
Exemplary embodiments of the present invention are directed to a passivation layer for a thermal ink jet printhead that is a corrosion and cavitation resistant thin film, is substantially atomically flat or has a controlled roughness, and is corrosion resistant.
In accordance with exemplary embodiments, a passivation layer for a thermal ink jet printhead is provided. The passivation layer is conformally disposed as an amorphous or pseudo-amorphous layer over a resistor by sputtering or other physical vapor deposition techniques and is in fluid contact with the ink in a thermal ink jet printhead. When substantially atomically flat, the surface roughness of the passivation layer is xe2x89xa650 xc3x85, preferably xe2x89xa620 xc3x85, and most preferably is xe2x89xa610 xc3x85. Alternatively, the passivation layer can have a controlled surface roughness wherein the controlled surface roughness is xe2x89xa7100 xc3x85.
The material of the passivation layer is disposed as an amorphous or pseudo-amorphous layer of small grain sizes, as small as the nanoscale. Exemplary materials for the thin layer display cavitation and corrosion resistant properties. Suitable materials include Co-based alloys and Fe-based alloys. The Co-based alloys can have 25-30 wt % Cr and optionally xe2x89xa65.0 wt % Fe. The Fe-based alloys can have xe2x89xa610 wt. % Co, xe2x89xa620 wt. % Cr, and xe2x89xa610 wt. % Mn. The Co-based and the Fe-based materials exhibit a cavitation rate of less than 7 mg/hour and preferably xe2x89xa64 mg/hour.